Time division duplex (TDD) wireless networks, such as Long Term Evolution TDD networks (LTE TDD), use a guard period (GP) between downlink (DL) and uplink (UL) transmission periods. In LTE, transmissions are subdivided into radio frames (10 ms) and subframes (1 ms). In TDD mode, subframes are dedicated to UL transmission, to DL transmission or a subframe is used to transit from DL to UL. In the latter case, the subframe is called special subframe and it starts with a part for downlink transmission, called DwPTS. DwPTS is followed by the guard period, which is followed by a part for uplink transmission, called UpPTS. In the following, DL transmission periods, i.e., DL subframes and DwPTS, are simply called DL frames and UL transmission periods, i.e., UL subframes and UpPTS, are simply called UL frames. In a cellular system, the downlink refers to a communication from a base station (also referred to as eNB) of a cell to user equipment (UE), e.g. a mobile terminal; whereas the uplink refers to a communication from the user equipment to the base station.
In TDD, a base station of one cell in a cellular wireless communication system potentially interferes with a base station of another cell. For example, the signal transmitted by a base station of one cell during a downlink frame may interfere with a signal received at another base station of a neighboring cell during an uplink frame. This type of interference is also known as inter base station interference (eNB-eNB interference). In order to mitigate inter base station interference TDD networks are usually synchronized, so that DL transmissions and UL transmissions in different cells occur at the same time. However, due to the propagation delay of the transmission from one base station to another inter base station interference may even occur in synchronized networks during the uplink frame following the downlink frame during which the transmission took place.
The guard period needs to be large enough to avoid interference during the uplink frame of neighboring base stations. However, the guard period takes away resources for the transmission of payload data, i.e. the time resource. Therefore, a guard period which is as short as possible would improve the efficient usage of the time resource. In general, a minimum length of a guard period would improve performance of the wireless system.
Let us assume that a base station sends data during the entire downlink frame. Then, to be able to receive all data transmitted during the downlink frame, the user equipment needs to account for the propagation delay from the base station to the user equipment, i.e. the user equipment needs to stay in receive mode after the end time of the downlink frame has been reached for a further time period covering at least the propagation delay from the base station to the user equipment.
This propagation delay from the base station to the user equipment is denoted by TpropeNB-UE. The user equipment requires a certain switching time TswitchUE to switch from a receive mode to a transmit mode. Assuming, that the channel remains reciprocal, the propagation delay for transmissions from the user equipment to the base station will also be TpropeNB-UE. To be able to receive by the user equipment all data transmitted in the downlink frame and by taking into account the effect of switching time at the user equipment and the propagation delay the guard period must have the minimum duration ofGPmin=2*TpropeNB-UE+TswitchUE.
A guard period between downlink frames and uplink frames which is determined according to this formula does not take the effect of interference between neighboring base stations into account. This kind of interference can be severe since base station transmit power is usually high and base station to base station (eNB-eNB) radio channels might have good propagation conditions due to large antenna gains and possible line-of-sight conditions resulting, for example, from above roof-top deployment of base stations. Thus, the uplink frame may strongly be affected by this kind of interference which may significantly reduce the system's performance.
Besides accounting for the transmission delays and switching times, it would be desirable that the guard periods also helps to avoid occurrence of inter base station interference in the uplink frame. Direct inter base station interference (eNB-eNB interference) could only be mitigated entirely if the guard period would cover the inter base station signal propagation delay from the most distant eNB of the network. Such a large guard period however would degrade the system's performance.
A typical way to determine a guard period between a downlink frame and an uplink frame in a TDD cellular wireless communication system is to rely on experience of system designers. The system designer uses his experience to configure the guard period manually.